Open you video with Aegisub:
1. Mark the start and end position of your subtitle
2. Export it to *.srt
3. Get the latest MP4Box, install MeGUI to get the last one, my version is "MP4Box - GPAC version 0.7.2-DEV-rev357-g7115eeb8-master"
4. Convert the .srt to .ttxt with the command below:

5. Open your "converted_subtitle.ttxt" in a text editor (Notepad++, Geany) and edit the "fontName" with the font you want, same for "fontSize" and save it.
6. To mux it in to your mp4 use the command below: